Process and method for quenching incipient gas-air explosions



PROCESS AND METHOD Fon QUENCHING INCIPIENT GAS-Am ExPLosIoNs Filed OCf.20, 1967 9, 1969 D. w. MITCHELL ET AL 3 Sheets-Sheet l L arm www .l VW.M6 .nur maMom .'l Nn .lll A/AN .Zn Mwww DEEJ mmm l ROGER WILL/AMSATTORNEY Dec. y9. 1969 D. w. MITCHELL ETAL 3,482,637

PROCESS AND METHOD FOR QUENCHING INCIPIENT GAS-AIR EXPLOSIONS Filed001.. 20, 1967 3 Sheets-Sheet 2 /FLAME 2o 60 D. UJ C1 m 0- P 3 E g LuPRESSURE j w |0- d3oLL D. z F/G. 8 D E l 4 s 25 50 DISTANCE OF QUENCHINGDEVICE FROM WORKING FACE- FEET PROCESS AND METHOD FOR QUENCHINGINCIPIENT GASA1R ExPLosroNs Filed OG'I.. 20, 1967 9, 1969 n. w. MITCHELLET AL 3 Sheets-Sheet 3 |50 DUST' DISPERSION TIME FOLLOWING GAS IGNITIONMILLESECONDS IOO United States Patent O 3,482,637 PROCESS AND METHOD FORQUENCHING INCIPIENT GAS-AIR EXPLOSIONS Donald W. Mitchell, Bethel Park,and Edwin M. Murphy,

Edward M. Kawenski, John Nagy, and Roger P. Williams, Pittsburgh, Pa.,assignors to the United States of America as represented by theSecretary of the Interior Filed Oct. 20, 1967, Ser. No. 677,511 Int. Cl.A62c 35/02; F42b 3/00; G01t 1/16 U.S. Cl. 169-1 15 Claims ABSTRACT OFTHE DISCLOSURE Gas-air explosions, such as those occurring in coalmines, are quenched by Vsensing the ultraviolet emissions of thedeveloping llame front and dispersing a powdered quenching agent such aspotassium bicarbonate into the immediateI llame environment in responseto the sensed radiation.

Background of the invention A gas explosion hazard continuously existsin most coal mines. Methane is usually occluded in the coal seam beingworked, a ton of unmined coal containing as much as 1000 cubic feet ofthe gas. Neighboring strata Often contain additional methane. The rateof methane release in or near the working face increases with the rateof face advance and with the proportion of fine coal produced. Methanefrom neighboring strata enters through cracks and crevices thatgenerally form in the strata above and below the coal seam as it ismined.

The potential hazard of a gas exploson in a coal mine is greatest at ornear the Working face. Here, all factors tend to intensify the hazard;gas emission is greatest, concentration of men and electrical equipmentis highest, space is limited, ventilation is difficult and visibility ispoor. About 70% of reported coal mine gnitions occur at the working faceand these ignitions are usually caused by frictional sparks created bycutter bits striking pyrites or hard rock.

At the present time the ignition hazard is controlled primarily byproviding good ventilation at the coal face so as to dilute the methanesuiciently with air to obtain a non-ignitab'e mixture Control offrictional sparking from cutter-bit action is probably impossible. Infact, modern mining methods tend to increase this hazard by increasingthe loan on and the speed of the cutter bits.

It has been proposed to minimize the explosion hazard in coal mines bydetecting and suppressing a potentially dangerous explosion immediatelyfollowing ignition and before the explosion has reached a destructivelevel, One such system is disclosed in the Glendinning et al. Patent,U.S. 2,693,240. ln this patent, the incipient explosion is detected bysensing the abnormal pressure rise characteristic of a developingexplosion. A signal is generated by the abnormal pressure rise and thissignal is used to trigger the explosive distribution of an extinguishingmaterial such as methyl bromide.

This type of device has many serious disadvantages when used in a coalmine. Use of a pressure rise measurement to detect a developingexplosion introduces an inherent lag or delay into the system. Forexampe, in a methane-air explosion in which the methane concentration isabout 9-l0%, or roughly stoichiometric, the llame front outruns thedeveloping pressure wave for a short period of time. Methane ignitionscharacteristically have a time lag between initiation and pressuredevelopment. This time lag has been found to range from about 3,482,637Patented Dec. 9, 1969 ICC to 2000 milliseconds, being lowest forstoichiometric homogenous mixtures. The time lag increases with decreasein homogeneity of the mixture and is highest for rich concentrations.

In a typical coal mine gas ignition, men are often Working in theimmediate arca; sometimes within a few feet of the ignition source. TheGlendinning et al. device allows a pressure wave on the order of 2p.s.i.g. to develop before the exploison is quenched. Eardrums ruptureand concrete-block walls are collapsed by a peak overpressure of 2 psig.A pressure wave of this magnitude will also raise considerable amountsof coal dust into suspension which provides additional fuel for theilame front and increases the severity of the explosion.

The explosive shattering of a rigid, sealed container offire-extinguishing materia. can create a pressure pulse well in excessof Z p.s.i.g. This pressure pulse not only can damage the hearing of aman in close proximity to the rupturing container but aso tends to raisecoal dust into suspension. Fragments of the shattered container tend toact as missiles having high initial velocities presenting a severehazard to anyone in the immediate area. Another hazard is from noisedamage. This is particularly severe in the relatively confined areawhich typies the working face of a coal mine.

It has also been proposed to substitute a radiation detector, such as aphotomul'iplier tube, as the explosion sensing device in place of thepressure rise detector of the Glendinning et al device. Exemplary ofthis approach is the Mathisen Patent, U.S. 2,869,647. Such a radiationdetector offers increastd sensitivity and decreased reaction time ascompared to a pressure-rise device,

A radiation-sensing explosion detector, when used in the environment ofa coal mine, is completely unsatisfactory if it responds to non-llamerelated radiation. Radiation sources present in coal mines include theincandescent lights used on mining machines and by miners, gamma rayemission produced by cosmic radiation and by radon gas, electric arcsand by frictional sparks produced by the cutter bits. A satisfactorydetector must discriminate between these spurious radiations and thoseproduced by a llame front.

The minimum design criteria. for a device capable of suppressing coalmine explosions are as follows:

(l) The device should be a self-contained unit capable of being mountednear the face end of a mining machine. The ei'liciency of llamequenching decreases rapidly with an increase in distance from theignition source.

(2) The device must be suiciently rugged to withstand the continualvibration and shock Of a mining machine.

(3) The llame detector should sense flame radiation anywhere within asolid angle of at least This requirement is based on typicalmachine-face crosssectional area ratios and on observations of flamespread in stratified gas-air bodies.

(4) The developing flame should be detected by radiation in theultraviolet region in order to avoid interference with other normallypresent coal mine radiations. Response time of detectors sensing anincrease in pressure or temperature is too slow for subsequent quenchingof burning methane.

(5) Flame must be detected and suicient flame quenching agent must tillthe working face within 50 milliseconds after gas ignition.

(6) The flame quenching agent must be uniformly dispersed. Non-uniformdispersion will inhibit but will not quench developing explosions.

(7) The llame quenching agent must not present a hazard to workmen. Manycommon fire extinguishing materials cause severe respiration and eyedamage in the concentrations necessary to quench an explosion.

(8) The means for dispersing the quenching agent should neither raiseadjacent coal dust into suspension, eject flying objects nor damage thehearing of workmen adjacent to it.

The present invention comprises a process and apparatus, which can beoperated in close proximity to workmen without hazard, for sensing andquenching developing explosions before the explosion can cause injury ordestruction. The apparatus comprises a radiation sensor which isresponsive only to emmissions characteristic of a methane-air ilameoperatively coupled to a flame quenching agent dispersal system havingan extremely rapid response time and which produces a very low pressureshock wave.

Accordingly, it is a primary object of this invention to quenchexplosions without hazard to adjacent personnel.

It is another object of this invention to provide apparatus capable ofdetecting a developing explosion in a lighted environment andsuppressing that explosion by dispersing a flame quenching agent intothe explosive environment.

A specific object of this invention is to quench gas-air explosions incoal mines by sensing only the ultraviolet emissions characteristic of amethane ame thereby activating dispersing means to distribute flamequenching agent to suppress the explosion Within 50 milliseconds oforiginal ignition.

Description of the invention Certain embodiments of the invention willnow be described -With reference to the accompanying drawings in which:

FIG. 1 is a general schematic representation of the explosion quenchingsystem.

FIG. 2 is a partial sectional view of a quenching agent dispersingcontainer having linear explosive means for distributing the quenchingagent.

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 2.

FIG. 4 illustrates another embodiment of the -quenching agent dispersingcontainer.

FIG. 5 is an alternative embodiment of the device of FIG. 2incorporating integral protective means for the quenching agent charge.

FIG. 6 illustrates the attitude of the expended device of FIG. 5.

FIG. 7 is a view of a coal mine showing the orientation of the explosionquenching system relative to a mining machine and the working face.

FIG. 8 is a graphical representation of pressure development and ameextent relative to location of the quenching agent dispersing container.

FIG. 9 illustrates the relationship of flame extent and pressuredevelopment to the timing of flame quenching agent dispersal.

FIG. l0 shows the spectral distribution for a methaneair-llame.

In order to prevent injury to workmen and damage to structure andequipment, a gas-air explosion must be quenched within about 50milliseconds after ignition. This time period is the total of theindividual time intervals for sensing tiarne, activating the quenchingagent dispersing system, dispersion of the flame quenching agent andsubsequent chemical and physical flame-extinguishing reactions. As shownin FIG. 9, at time intervals greater than about 50 milliseconds, eitherthe volume of flame is appreciable or explosion pressures are suicientlyhigh to cause injury and destruction.

As was expected, stoichiometricgas-air mixtures were the most ditlicultto quench. As shown in figure 9, satisfactory explosion quenching ofsuch mixtures was achieved only when the time interval between ignitionand actuation of the dispersing system for the quenching agent was nomore than 20 milliseconds. When the dispersing system was actuatedwithin 10 milliseconds, llame extension was less than 5 feet and nomeasurable pressure developed. When actuated 20 milliseconds afterignition, flame extended about 5 feet and a maximum pressure of about0.6 p.s.i.g. developed. When actuated 50 milliseconds after ignition,llame extended a suicient distance to have enveloped and burned theminers.

It is thus clearly evident that a satisfactory ydetector must have anextremely fast response time; much less than 50 milliseconds andpreferably much less than 10 milliseconds. Referring now to FIG. 1, theexplosion suppression device of this invention comprises a radiationsensor 1 which transmits a signal to photon counteramplifier 2 viaconnection 3.

Radiation sensor 1 comprises a gas lled photodiode or Geiger-Mller typetube and operates in the photoelectric gas-gain mode. A photon-counterwith an adjustable threshold of sensitivity, such as that disclosed inthe Friedman Patent, U.S. 2,715,195, is an example of a device which maybe employed as the sensor. Photons having an energy greater than thatequivalent to a wavelength of about 3000 Angstroms (A.), pass throughthe contained gas and strike the electrode surface producingphotoelectrons. The photoelectrons formed migrate under influence yof anelectric field within the tube creating secondary ionization andresulting in an electron avalanche that is fed into photoncounteramplifier 2 as a pulse of current. Photon counteramplier 2 is amultivibrator pulse shaper that iirst broadens each pulse of currentfrom the tube so that subsequent electronic components would be capableof sensing what otherwise would be a signal too short in duration forreliable operation of the pulse counting circuitry. The pulse countingcircuitry in photon counteramplilier 2 is so constructed that asubsequent triggering circuit is activated only when the photon pulserate reaches a preset level equivalent to 4 or more pulses per halfsecond. Photon counterampli- Iier 2 circuitry also is so constructedthat the accumulated count is nulliiied each second; this nullificationor forgetting circuit reduces the sensitivity of the sensor to noname,high-energy radiation sources such as gamma rays from cosmic rays andradon gas (the circuits and tube are unaffected by alpha and beta rays).For example, the gamma-ray activity associated with cosmic rays in coalmines seldom exceeds l pulse per second; should the electronic circuitryaccumulate those pulses in photon counteramplier 2 then it is possiblefor the device to be activated in time by a cosmic rather than amesource. In summary, though the detector tube responds to all radiationshaving al wavelength below about 3000 A. the output from photoncounteramplier 2 is restricted in such a manner that the llame detectorunit as a whole can be considered to be responsive only to radiationhaving a wavelength between about 2000 to 3000 A. In other aspects,photon-counteramplitier 2 is a standard component well known in the art.One example, of a commercially-available radiation sensor which may beused in this device is the DP 28/A flame detector manufactured byMelpar, Inc., Arlington, Va.

When the photon rate in photon counterampliiier 2 reaches the abovediscussed preset level equivalent to 4 pulses per half second atriggering circuit is activated that sends a signal of about 221/2 voltsDC through connection 4 which energizes shot-tiring condenser 5. Thecondenser discharges through wires 6 and triggers an electrical blastingcap 7 which in turn ignites the explosive charge used to distributequenching agent from dispersing system 8. Construction and operation ofthe dispersing system will be detailed later.

FIG. 10 illustrates the spectral distribution for a methane-air flameand indicates the relative intensities of the spectral radiations. Ascan be seen from the figure, the

flame radiates rather strongly in the 2000 to 3000 A. range. The majorspectral peak between 4000 and 5000 A. cannot be discriminated from theradiation produced by incandescent lights used on mining machines and byminers. These lights do not emit radiation below about 4000 A. Theenergy level of a methane-air flame below about 4000 A. is notsufficient to permit use of photoconduction cellsy photocells andphotomultipliers when these devices are coupled with the necessaryoptical filters to block out transmission of visible light. Infrareddetectors would require filters to avoid interference with incandescentradiation and would respond more to the hot combustion gases and vaporsproduced by a flame and by heated lubricants on the mining machinerather than to the llame itself. Another disadvantage inherent in usinginfrared detectors is that water vapor tends to strongly absorb infraredradiation while it is effectively transparent to ultraviolet radiation.

Of the quenching agents studied, potassium bicarbonate having an averageparticle diameter of about 16 microns, had the best combined qualitieswith respect to toxicity, effectiveness and cost. The KHCO3 particlespreferably are siliconized to reduce agglomeration and moistureadsorption. It is also preferred to admix with the potassium bicarbonatea small amount of granular, moisture-indicating anhydrous calciumsulfate or other indicating desiccant to reveal the presence ofmoisture.

Conventionally-used explosion suppression substances, such as methylbromide and the like, were ruled out as being unsatisfactory in thisapplication. Methyl bromide is a severe skin and respiratory irritantand can cause death upon short exposure to the concentrations requiredto suppress a developing explosion. In addition, methyl bromide, with aboiling point of about 3.5 C., must be contained under pressures ofabout l0 to 25 p.s.i.g. at the conditions encountered in ordinary coalmines. Explosive rupturing of a container at those pressures generatesan intolerably great shock wave as well as projecting parts of theshattered container at velocities sufficient to cause severe injury.

The comparative effectiveness of the quenching agents studied is shownin the following table:

TABLE 1 [Effectiveness of predispersed agents in quenching ignition of a1,300- cubic-ioot homogeneous 9.5% gas-air body] Average Maximum 1Pressures below 0.2 p.s.i.g. cannot be determined with present ECMequipment. l

2 Quantity probably exceeded that capable of being dispersed by 20 feet;

of 50-grain detonating cord.

In order to quench developing explosions in a relatively confined areasuch as the working face of a coal mine, the flame quenching agent mustbe dispersed as a relatively uniform dust cloud into the flammableatmosphere. High speed movies show that non-uniformly distributed dustinhibits but will not quench developing explosions. Distribution of auniform cloud of flame quenching agent must be accomplished within avery short period of time, preferably in less than about 40milliseconds, yet the peak overpressure resulting from such distributioncannot exceed about 2 p.s.i.g. and preferably should be held well below1 p.s.i.g. in order to avoid injury to workmen. Conventionalexplosivelyoperated dispersing systems develop pressures far in excessof 2 p.s.i.g.

Devices of this invention which meet those requirements are illustratedin FIGS. 2 to 6. FIG. 2 is a partial sectional view of a device fordispersion of a particulate, flame quenching dust such as potassiumbicarbonate. FIG. 3 is a cross-section of that same device taken alongline 3--3 of FIG. 2. As shown in the gures, the device comprises arelatively rigid, elongated, trough-like container 10, open at the topand preferably having a uniform cross section. Container 10 may behemispherical in cross section as is shown in FIG. 3, or it may be V-shaped. Flat-bottomed or channel-shaped containers may be employed butthese shapes provide less efficient dispersion of the quenching agent.Disposed within and extending the length of container 10 is thin-walled,easily rupturable bag 11 normally filled With a powdered,flame-quenching agent such as potassium bicarbonate 12. Bag 11 must beconstructed from a Water-impervious material. Particularly preferredbecause of its excellent resistance to puncture and to abrasion isurethane vinyl laminate. This flexible cloth meeting MIL Spec. MIL-C-43006B consists of 2 plies of calendar urethane around a single ply ofZ-denier nylon mesh; the thickness of the finished cloth is betweenabout 4 to 7 mils. To insure that this bag will be ruptured readily bythe relatively Weak pressure generating means 13, the edges of the clothare sealed by electronic welding only, the weld being a simple pinchseal having a width of about 1z-inch. Another suitable material for bag11 is a relatively brittle film such as Mylar (polyetheleneterephthalate) having a thickness of about 1 mil. Other plastics havinggreater elasticity, such as polyethelene, polypropylene, vinyl acetateand the like, require greater force for effective dispersion of thequenching agent and so produce a larger shock Wave. Container 10 mayhave open ends or may have suitable plate-like end pieces (not shown).

Extending inside and along the lower side of bag 11 is pressuregenerating means 13. This pressure generating means preferably comprisesa linear explosive charge such as detonating cord. The detonating cordis secured in the lower side of the bag by plastic rings or plasticsleeve 14 which is glued or otherwise permanently attached along thelower side of bag 11 and is preferably constructed of a brittle plasticfilm such as Mylar. Triggering means 15, which preferably comprises anelectrical blasting cap, is located at one end of the device and isactivated by an electrical pulse produced by the sensing means (notshown). Wires 16 connect the triggering means to the sensor. Protectivegrid or screen 17 surmounts container 10 and protects bag 11 frompuncture due to flying coal particles and the like. The protectivescreen may comprise a heavy wire mesh or similar maferial. An expandedmetal grid having l-inch diamond shaped holes was found to besatisfactory.

The relationship of linear explosive to quenching agent must becarefully balanced in order to insure rapid and thorough dust dispersioninto the flammable atmosphere without creating a harmful pressure wave.For example, in the device of FIG. 2, detonating cord containing 50grains of PETN per foot effectively dispersed 4 pounds of dry potassium`bicarbonate per foot of detonating cord and produced an overpressure ofabout 0.8 p.s.i.g. at a distance of 4 feet from the device. Lowerstrength detonating cord was less satisfactory; pressures as high as 1.5p.s.i.g. developed in trials with 25- and 30-grain cord and 2 pounds ofKHCO3 per foot of cord.

FIG. 4 illustrates another embodiment of the quenching agent dispersingdevice. The device comprises a container 21, conveniently of tubularshape and having closed ends 22. It is constructed of a relativelyrigid, frangible material such as acrylic tubing. The upper portion 23of the container is scored so as to break into relatively smallfragments upon the application of a sudden internal pressure of lessthan 2 p.s.i.g. and preferably on the order of 1/2 p.s.i.g. Container 21is normally iilled with a powdered, iame quenching agent.Pressuregenerating means similar to that employed in the device of FIG.2 extend the length of container 21 and terminate externally in a pairof lead wires 24.

Another alternative embodiment of the quenching agent dispersing deviceis shown in FIGS. 5 and 6.. The outer protective cover of the devicecomprises two symmetrical elongated sides or wings 31 and 32 flexiblyjoined or hinged at their bottom side such as by hinge 33. The two wingsare constructed of a rigid sheet metal or like material and are normallyheld in a closed position by clips 34 along their top abutting edges.End pieces 3S and 36 in conjunction with wings 31 and 32 form a normallyclosed container. Disposed within the normally closed container is aneasily rupturable bag 37 containing quenching agent and havingassociated therewith pressure generating means 38, activating means 39with lead wires 40 similar to those of the device of FIG. 2.

In operation, activation of the pressure generating means forces therelease of clips 34 allowing wings 31 and 32 to move outwardly pivotingon hinge 33. Quenching agent contained in bag 37 is rapidly dispersedupwardly and to the side by the generated pressure. Attitude of theexpended device is shown in FIG. 6.

The quantity of ame quenching agent or dust dispersed must be sufcientto provide a concentration capable of almost instantaneously quenchingflame. It is theoretically necessary to provide about 5 pounds of 2-micron potassium bicarbonate in a predispersed cloud in order to quenchignitions in a 1300 cubic foot homogeneous mixture containing methane inair. This corresponds to a concentration of about 0.06 ounce per cubicfoot. Experimental results showed however, that about 13 pounds ofpredispersed potassium bicarbonate was actually necessary to quench anignition at those conditions. Dispersal after ignition requires yethigher concentrations of the quenching agent in order to be eective asis shown by Table 2. This table shows the minimum quantity of potassiumbicarbonate dust required to quench ignitions in a 1300 cubic foothomogeneous mixture containing 10% methane in air for various dustdispersing systems. Ignition of the gas and dispersion of the dust waseiectively simultaneous in all cases.

TABLE 2 Potassium Bicarbonate Particle As may be seen from the table,very finely divided (2-micron) potassium bicarbonate is somewhat moreeffective than coarser (I6-micron) material as would be expected. At thepresent time however, the gain in eiiicency of the finely dividedmaterial is more than oITset by its increased cost. Use of a protectivescreen over the device also tended to decrease efficiency as shown byexperiment 4. A minimum of about 0.5 ounce of potassium bicarbonate percubic foot was found to be necessary to insure quenching ofstoichiometric gas-air ignitions.

Location of the quenching devices relative to the point of ignition isquite important. FIG. 8 shows the relationship of pressure developmentand flame extent to the distance from the working face (source ofignition) in a coal mine. As can be seen from the graph, both pressuredevelopment and flame extent increased rapidly as the distance betweenthe quenching devices and the ignition source was increased beyond 6feet.

Orientation of the quenching devices was also found to be important.Devices oriented with their long axis parallel to the axis of the entrywere found to be much more effective than those which were orientedperpendicular to the axis of the entry. Height of the quenching deviceabove the iioor of the entry had no signicant eiect on quenchingefficiency.

FIG. 7 illustrates the use of this invention in a typical coal mine.Entry 51 is conventionally on the order of about 20 feet wideand isdened by side walls 52 and 53 and working face 54. Cross-cuts 55 areconventionally provided to allow ventilation and prevent gas build-up.Mining machine 56 rips coal from the working face and transports itbackward to a conveyor belt or other transport system (not shown).Radiation sensor 57 is preferably mounted near the headlights of themining machine where it can scan the entire working face and where it isconvenient for the miner to clean the protective lens on the detectortube. At this location, the sensor will be subjected to maximumaccelerations of about 6 G at a predominant fundamental frequency ofabout 18 cycles per second. A pair of quenching devices 58 arepreferably mounted one on each side of the machine. The quenchingdevices are oriented with their long axes parallel to the axis of theentry with their forward ends within about 6 feet of the working face.Radiation sensor 57 is operatively connected to the quenching devices bymeans of tie-lines 59.

During the mining operation, methane is released from the working faceas coal is ripped from the seam by the mining machine. Methaneconcentration is normally held at safe levels by dilution withcirculating ventilation air. If methane concentration at the workingface reaches the flammable range, an ignition and resulting explosion isvery likely due to frictional sparking of the cutter bits. At this time,the initial llame is detected by the radiation sensor which triggers therelease of flame quenching agent and suppresses the developingexplosion.

While the invention has been illustrated as being primarily applicableto the suppression of explosions in coal mines, it is readily evidentthat it is adaptable to any other type of operation which has similarhazards. For example, the invention would find ready use in suchoperations as servicing and arming aircraft, particularly in closedhanger areas.

It will be understood that a number of adaptations and variations of thedisclosed invention are possible without departing from its spirit orscope.

What is claimed is:

1. A device for quenching incipient explosions of gasair mixturescomprising in combination flame sensing means having an effectivesensitivity only to ultraviolet radiation above about 2000 A. andcapable of producing a signal in response to said ultraviolet radiationwithin about 10 milliseconds of the ignition of said gas-air mixture, alongitudinallyextended container having an upper surface adapted torupture or open upon the application of a sudden internal pressure, saidinternal pressure being limited to a level below that which causesphysical injury to personnel adjacent thereto, and normally holding aquantity of nely divided particulate flame quenching agent, pressuregenerating means disposed within the lower portion of and extending thelength of said container and triggering means operatively coupled withand capable of activating said pressure and said triggering meanscomprise an electrically activated detonator.

4. The device of claim 3 wherein said longitudinally extended containercomprises a relatively rigid troughlike channel having disposed thereinand extending the length thereof a thin-walled, relatively brittleeasily rupturable bag surrounding and protecting `said potassiumbicarbonate.

5. The device of claim 3 wherein said longitudinally extended containercomprises a closed tube of a relatively rigid frangible material andhaving a portion of its upper surface scored `so as to break andfragment upon the application of a sudden internal Vpressure of lessthan about 2 p.s.i.g.

6. A device of claim 3 wherein said longitudinally extended containercomprises two symmetrical elongated sides ilexibly joined at their loweredges and having their upper edges yieldably clipped together and havingdisposed therein and extending the length thereof a thinwalled,relatively brittle, easily rupturable bag surrounding and protectingsaid potassium bicarbonate.

7. A device comprising a longitudinally extended container having arelatively uniform cross section and adapted to rupture or open along aportion of its upper surface upon the application of a sudden internalpressure of less than about 2 p.s.i.g. and having uniformly disposedwithin said container a quantity of nely divided particulateflame-quenching agent, pressure generating means disposed within thelower portion and extending the length of said container and triggeringmeans adapted to activate `said pressure generating means in response loan external signal.

8. The device of claim 7 wherein said pressure generating means comprisedetonating cord and said tri1- gering means comprise an electricallyactivated detonator.

9. The device of claim 8 wherein said container comprises a relativelyrigid trough-like channel having disposed therein and extending thelength thereof a thinwalled, relatively brittle, easily rupturable bagsurrounding and protecting said flame quenching agent.

10. The device of claim 8 wherein said container cornprises a closedtube of a relatively rigid frangible material and having a portion ofits upper surface scored so as to break and fragment upon theapplication of a sudden internal pressure of less than about 2 p.s.i.g.

11. The device of claim 8 wherein said container comprises twosymmetrical elongated sides tlexi'bly joined at their lower edges,having their abutting upper edges yieldably joined and having disposedtherein and extending the length thereof a thin-walled, relativelybrittle,

easily rupturable bag surrounding and protecting said llame quenchingagent.

12. The method of preventing explosions of gas-air mixtures whichcomprises detecting only the ultraviolet radiation having a wavelengthabove about 2000 A. produced by a gas-air llame, producing a signal inresponse to said detected radiation, explosively distributing a linearcharge of finely-divided, particulate flame quenching agent throughoutthe area adjacent to said llame in response to said signal, maintainingthe maximum pressure wave developed during said explosive distributionbelow the level at which coal dust is raised into suspension and belowthe level which causes physical injury to workmen, quenching said llameby means of physical and chemical reactions between said distributedflame quenching agent and the developing flame front and accomplishingsaid steps of detecting radiation, distributing said flame quenchingagent and quenching said llame within 50 milliseconds of the ignition ofsaid flame.

13. The process of claim 1`2 wherein said flame quenching agent ispotassium bicarbonate.

14. The process of claim 13 wherein the amount of said potassiumbicarbonate distributed is sutllcient to provide a relatively uniformconcentration of at least about 0.5 ounce per cubic foot in the areaadjacent to said flame.

15. The process of claim 14 wherein said maximum pressure wave developedis less than about I/2 p.s.i.g.

References Cited UNITED STATES PATENTS 1,708,869 4/1929 Buddecke 169--282,373,819 4/1945 Eaton 169-28 2,869,647 1/1959 Mathisen 169-4 3,196,2737/1965 Abromaitis 'Z50-83.3 X 3,258,423 6/1966 Tuve et al.

3,320,881 5/1967 Brett et al. 102-23 OTHER REFERENCES Gas FilledUltraviolet Detector Warns of Fires and Explosions, Electronics, by D.H. Howling and R. C. Roxberry, May 26, 1961.

M. HENSON WOOD, JR., Primary Examiner MICHAEL Y. MAR, Assistant ExaminerU.S. Cl. X.R.

